Age-related changes in levels, actions, and secretion of the somatomedins and related growth-promoting hormones are being studied. Fisher 344 rats from the NIA barrier-protected colony at Charles River are used for all studies. Serum content of active somatomedin is measured by a cell growth bioassay. Actions of somatomedins on muscle are measured as 3H-aminoisobutyric acid uptake by diaphragm, soleus, and omohyoideus muscles. Secretion of somatomedin is measured by assay of serum following injection of growth hormone.